One of the functions of a Global Navigation Satellite System (GNSS) receiver is to perform “acquisition.” That is, its acquisition unit is responsible for finding which GNSS signals are visible to the receiver and estimating the frequency shift and code delay of those signals in order to track them. To find these signals and determine their frequency shift and code delay the acquisition unit has to go through the frequency and code search space.
In general, the larger the search space, the longer it takes to acquire a signal. The frequency search space typically depends on how large the frequency shift can be. The frequency shift of the received signal, with respect to the signal nominal frequency can be caused by different factors: Doppler induced by the satellite movement with respect to the receiver, Doppler induced by the receiver movement, and the frequency shift caused by the clock used in the receiver.
Conventionally, when a receiver has information about its current position and the current time as well as valid almanac data, it can use this information to predict which satellites will be visible and to estimate Doppler induced by satellites. This allows the acquisition function to search only for the visible satellites and reduces the frequency search space to an extent. Such an acquisition is known as acquisition with initialization.
Standards for aviation GNSS receivers define specific requirements regarding the maximum time the receiver has to perform the first position fix when an acquisition with initialization is performed. This, in turn, imposes limits on the time required to acquire the initial 5 satellites (needed to obtain a position fix with integrity) by the acquisition function. When GNSS signals have increased code lengths and modulations with narrower autocorrelation functions the total search space increases.
For the reasons stated above and for other reasons stated below, it will become apparent to those skilled in the art upon reading and understanding the specification, there is a need in the art for methods and systems to reduce the frequency search space and speed up the acquisition process.